The Global Positioning System (GPS) includes 24 satellites orbiting the earth at around 20,000 km. The 24 satellites are distributed so that four satellites reside in each of six orbital planes which roughly form north-south orbits at around 55 degrees inclination from the equator. Each satellite traverses its orbit in approximately 12 sidereal hours. The satellites use spread spectrum techniques to repetitively transmit their versions of the current time, their current ephemeris data, and their identification numbers (SVIDs).
Roving GPS receivers receive these transmissions near the earth's surface and resolve their locations based upon the data conveyed in the satellite transmissions and upon characteristics of the received signals. The resolution of a roving receiver's location requires a pseudorange measurement and a pseudorange rate measurement to be determined. The pseudorange measurement corresponds to a propagation delay of a transmitted signal traveling between the satellite and the roving receiver, and the pseudorange rate measurement corresponds to the signal's Doppler. As implied by the "pseudo" prefix, these measurements may not be accurate because they are determined using the roving receiver's internal time base which is not synchronized with the satellite'stime base. Nevertheless, commercially available roving GPS receivers can autonomously determine their accuracy to within 100 meters or less.
However, many location applications require greater accuracy than that autonomously determined by commercial GPS receivers. Such applications include geographic information databases, emergency location services, and others. Greater accuracy can be achieved using a technique known as differential GPS. Generally, differential GPS uses a stationary master control station positioned at a known location to monitor GPS satellite transmissions. The master control station uses its known location to perform the inverse of the location resolution process to determine what pseudorange and pseudorange rate measurements should have been detected at the master control station to yield the correct location. The master control station generates measurement corrections once the appropriate measurements are determined. Positioning accuracies of less than five meters and even less than one meter in certain situations can be obtained when the autonomously determined pseudorange and pseudorange rate measurements for a roving receiver within a few hundred kilometers of the master control station are corrected by the differential GPS data.
Conventional master control stations tend to be complex, inflexible, and expensive pieces of equipment. Their complexity is due, at least in part, to the inclusion of more channels than are included in conventional roving GPS receivers. More channels are required so that differential GPS correction data may be provided at all times for the maximum number of satellites which can be in view of the master control station. Conventional master control stations tend to be inflexible and expensive because only a few are required to support an entire population of roving GPS receivers. Consequently, master control stations have not achieved the pricing, flexibility of use, and reliability advantages which have been achieved by mass market roving GPS receivers.